Entrainment in Cumulus Clouds: What Resolution is Cloud-Resolving?

Abstract

Systematic numerical experiments were conducted to determine the spatial resolution required to resolve a moist thermal show convergence at a scale proportional to the smaller of the initial thermal diameter D0 and a buoyancy length scale Lbuoy. The buoyancy length scale Lbuoy = ΔT0/ΔΓ (ΔT0 is the initial buoyancy excess of the thermal and ΔΓ is the ambient stratification) describes the maximum vertical displacement that can be induced against the stratification in the environment by buoyancy-driven pressure perturbations in the cloud and, thus, the maximum scale of eddies that cross the cloud boundary. For typical atmospheric conditions in which the cloud size D0 is larger than Lbuoy, numerical simulations of the mixing processes in cumulus clouds must resolve Lbuoy.